My two cents is the best way to think about unhooked highly selective holistic college admissions is a combination of meeting certain basic requirements in three areas–academics, activities, and personal characteristics-and then standing out in some additional way(s) on top of that. And they will pick a diverse mix of applicants who stood out to them in a variety of different ways. And actually, more often that will involve standing out in a way NOT related to your academic interests. Trying to stand out academically is actually a relatively hard path to choose.
In this framework, there may be some pretty specific things you need to check off, like a transcript that meets their standard expectations for a successful applicant from your sort of secondary school. But otherwise, they are not so much looking to deduct points for things you didn’t do but could have. They are going to focus on whatever highlights stand out to them among the things you actually did do.
So I really don’t think they do things like evaluate what you did in a particular summer and give you a point score for that. That doesn’t mean you should entirely ignore such periods, but they can be helpful in a myriad of ways. Maybe they help you do better on your classes. Maybe they help you do better in a school activity. Maybe you will develop your social and/or time management skills. Maybe you identify an interest to build on. That could be an academic interest, but it could be a career interest, it could be a service interest, it could in fact be a new hobby, or so on.
I think this becomes a question of understanding what sort of undergrad program you are actually applying for.
I agree there are some undergrad programs where they are admitting by specialty major, or possibly a specialty school of some sort, and they may be looking for qualifications specific to that specialty program. Even then, I do think coursework and test scores can go farther than some people realize, nor is it necessarily a good idea to have NO developed interests outside that specialty area. But having at least some experiences which help show why you are interested in that specialty program could be useful.
But again, other undergrad programs are exploratory in nature. And while some people might be strong candidates for both sorts of program, some people may well seem too narrowly focused, indeed too career focused, to an exploratory program, even if they would seem fine to a specialty program.
Unfortunately, in certain circles, there is a sentiment that every highly selective program is secretly looking for specialist kids, even if they say otherwise. I think that sentiment is wrong, and in fact I think it is contributing to why the admissions rates have gotten so low for some of these colleges. Of course some kids are applying without even being superficially qualified, but others are applying with very plausible qualifications, but also with an emphasis/focus that they think these colleges want to see, but the colleges themselves are not particularly interested in. And then they add to the 95% or whatever of applicants who apply to these colleges and get rejected.